1. Field of the Invention
The present invention relates generally to a terrestrial solar power system for the conversion of sunlight into electrical energy, and, more particularly to a solar cell array using IV-V compound semiconductor solar cells for unitary movement to track the sun.
2. Description of the Related Art
Commercially available silicon solar cells for terrestrial solar power application have efficiencies ranging from 8% to 15%. Compound semiconductor solar cells, based on III-V compounds, have 28% efficiency in normal operating conditions. Moreover, it is well known that concentrating solar energy onto a III-V compound semiconductor photovoltaic cell increases the cell's efficiency to over 37% efficiency under concentration.
Terrestrial solar power systems currently use silicon solar cells in view of their low cost and widespread availability. Although III-V compound semiconductor solar cells have been widely used in satellite applications, in which their power-to-weight efficiencies are more important than cost-per-watt considerations in selecting such devices, such solar cells have not yet been designed for optimum coverage of the solar spectrum and configured or optimized for use in solar tracking terrestrial systems, nor have existing commercial terrestrial solar power systems been configured and optimized to utilize compound semiconductor solar cells.
In the design of both silicon and III-V compound semiconductor solar cells, one electrical contact is typically placed on a light absorbing or front side of the solar cell and a second contact is placed on the back side of the cell. A photoactive semiconductor is disposed on a light-absorbing side of the substrate and includes one or more p-n junctions, which creates electron flow as light is absorbed within the cell. Grid lines extend over the top surface of the cell to capture this electron flow which then connect into the front contact or bonding pad.
One important aspect of a solar cell system is the physical structure of the semiconductor material layers constituting the solar cell. Solar cells are often fabricated in vertical, multijunction structures to utilize materials with different bandgaps and convert as much of the solar spectrum as possible. One type of multijunction structure useful in the design according to the present invention is the triple junction solar cell structure consisting of a germanium bottom cell, a gallium arsenide (GaAs) middle cell, and an indium gallium phosphide (InGa P) top cell.
Still another aspect of a solar cell system is the specification of the number of cells used to make up an array, and the shape, aspect ratio, and configuration of the array.
The individual solar cells are typically disposed in horizontal arrays, with the individual solar cells connected together in electrical series. The shape and structure of an array, as well as the number of cells it contains, and the sequence of electrical connections between cells are determined in part by the desired output voltage and current of the system.
Another aspect of terrestrial solar power system is the use of light beam concentrators (such as lenses and mirrors) to focus the incoming sunrays onto the surface of a solar cell or solar cell array. The geometric design of such systems also requires an appropriate solar tracking mechanism, which allows the plane of the solar cells to continuously face the sun as the sun traverses the sky during the day, thereby optimizing the amount of sunlight impinging upon the cell.
Prior to the present invention, there has not been an optimal combination of features relating to array design, solar cell receiver modules, and semiconductor device features suitable for terrestrial applications.